1. Field of the Invention
The present invention relates to autoloading firearms such as the M-16/AR 15 family, and more particularly, to firearms having an indirect gas operating system.
2. Background of the Invention
Most of the self-loading rifles designed during and after WWII have been gas-operated. Such systems differ from blowback and recoil systems in that the operating energy to cycle the bolt, extract spent ammunition and feed live ammunition comes from tapping expanding high-pressure gases at the barrel instead of from recoil forces. Examples include the U.S. M1 Garrand, M1A, and M1 Carbine, Simonov rifle (SKS), Automat Kalashnikov (AK), Swedish Ljungrnan, the late Eugene Stoner's AR10 and AR15 (M16), FN FAL, M60 and M249 Minimi machine guns.
Gas-operated designs vary in how the gas is tapped, and how gas energy is transferred to the bolt carrier. All of the weapons listed above, except the FAL, use rotary locking bolts which follow helical cam tracks in their carriers or receivers. In the Garrand, SKS and AK families, and most modern light to medium weight machine guns, gas impinges directly on operating rods fixed to the bolt carrier and located below or above the barrel. In the Ljungrnan and Stoner designs, a narrow steel tube carries the pressurized gas back to small operating cylinders on the bolt and carrier. In the M1 carbine and more recent designs such as Eugene Stoner's Amarlite AR-18, Australia's Leader, Singapore's Sterling-designed SAR rifle family, Korea's Daewoo rifle, and Heckler & Koch's 1998 G36, a small, low-mass tappet is the only moving part in contact with the gas. The tappet accelerates rapidly due to its low mass and imparts its momentum by striking an operating rod connected to or striking the bolt carrier.
U.S. Pat. No. 2,951,424 issued to E. M. Stoner on Sep. 6, 1960, discloses the M16 bolt and bolt carrier system and the gas operation thereof. This patent discloses a rifle utilizing a gas tube that extends from gas ports in the barrel, back into the receiver of the rifle and into a gas tube pocket or “key” attached to the bolt carrier.
U.S. Pat. No. 3,675,534, issued to P. C. Beretta on Jul. 11, 1972, discloses a gas-operated automatic rifle having a piston and stem inside a gas tube with the stem fixedly attached to the bolt carrier.
U.S. Pat. No. 4,358,986, issued to C. Giorgio on Nov. 16, 1982, discloses a gas-operated automatic rifle having a stationary piston and a segmented movable gas cylinder/operating rod assembly including a biasing spring.
U.S. Pat. No. 3,618,457, issued to A. Miller on Nov. 9, 1971, discloses a gas-operated rifle utilizing a gas-operated piston and rod assembly with the piston rod telescopically mounted over a stationary guide rod and being spring-biased.
U.S. Pat. No. 4,765,224, issued Aug. 23, 1988, to M. Morris discloses a modified M16 type of rifle utilizing an extended gas tube receiver on the bolt carrier which maintains telescopic engagement with the gas tube at all times during the firing cycle.
U.S. Pat. No. 4,475,438 to L. Sullivan, issued on Oct. 9, 1984, discloses an open-bolt gas-operated rifle with a short-stroke piston that kicks open the bolt carrier against a biasing spring, using a short-stroke piston movement.
One repeated criticism of the AR15/M-16 design is its direct gas impingement action. Propellant gas is tapped from the barrel and led through a tube backwards into the receiver of the piece, and into the bolt carrier itself. The gas pressure works against a piston which is a rearward-facing surface of the bolt. The pressure builds between that face of the bolt and the internal surfaces of the bolt carrier (‘directly impinging’ upon the bolt), forcing them apart. A cam translates that force into rotation of the bolt, so its lugs disengage with their mating surfaces, the bolt unlocks, and the bolt carrier can retract, carrying the bolt with it.
Users of the AR-15/M16 rifle are aware that the rifle requires a great deal of maintenance. Carbon that is vented back into the receiver from the gas tube essentially “plates” onto several portions of the bolt carrier and the interior of the receiver and after a few rounds are fired, the entire interior of the rifle is coated with a film of carbon. If allowed to build up, this carbon will eventually lead to stoppages, so the rifle must be cleaned frequently. A second problem with the AR-15 design is that the bolt carrier rides directly on the interior surface of the receiver. Because of the ever-present carbon fouling, most operators keep the bolt carrier well-lubricated, which turns it into a “dust magnet” unless the ejection port cover is kept closed at all times when the rifle is not in use. Excess dirt will cause the bolt carrier to eventually slow down and fail to fully chamber a round, which has necessitated the introduction of the “forward assist” button.
Over gassing is also a common complaint with the carbine length direct impingement action. Gas port and gas tube diameters are all based on a rifle length action which operates at a different pressure level, timing and volume.
The AR-18 solved some of the problems of the AR-15 by changing the gas system to a rod and piston arrangement that vents excess gases into the atmosphere just aft of the front sight/gas block. The piston is fixed; expanding gases drive the rod back into the bolt carrier, which rides on two fixed rods surrounded by recoil springs. The bolt carrier does not touch the interior surface of the receiver. Operating the bolt carrier on these two “action rods” gives minimum surface area for dust buildup or fouling.
Although in an indirect-impingement firearm, the operating rod moves the bolt carrier without depositing burnt-propellant crud in the receiver, the direct gas impingement system has the advantage of having fewer parts and less weight compared to a firearm that contains this gas in a cylinder up front of the receiver, and uses the pressure to move an operating rod. Long operating rods used with tappets or indirect impingement increase and spread out the moving mass, which tends to shift the point of balance when operated leading to dynamic problems.
Many previous attempts to provide a reliable gas piston system for the AR-15/M-16 weapon family system have suffered from dynamic problems related to off-center impact of the bolt carrier which causes the carrier to lift initially before moving backwards. This lift at the front of the carrier causes a corresponding dip at the rear of the carrier which causes the carrier to strike the lower receiver extension and cause excessive wear.